1. Field of the Disclosure
The present invention pertains to devices and methods for securing printing forms on a mountable surface, such as a cylindrical surface, and maintaining proper tension on the printing forms.
2. Description of Related Art
In a process for preparing from a photosensitive element a relief printing form, the printing form is often mounted around the circumference of a rotatable drum and heated to soften unpolymerized portions of the element for removal and create a relief surface of the printing form. During thermal processing, the drum is rotated and the heated printing form is placed in contact with a development medium, such as an absorbent surface, to remove the unpolymerized portions. It is desirable for thermal processing systems to accommodate printing forms of different sizes, and in particular to mount printing forms having different lengths around a circumferential surface of the drum. It is also desirable for the printing form to be securely mounted to the drum since during thermal processing the printing form needs to withstand the rigors of one or more cycles of heating and separating the development medium from the printing form. Defects can be induced in the resulting relief printing form if during thermal processing the still warm printing form lift, sags, or bends from the drum surface. The printing form can lift off the drum or sag and separate from the drum surface as the medium separates from the printing form, or as the printing form rotates on the drum. Uncontrolled separation of the medium and the lifting or sagging of the printing form while the printing form is still hot bends the element and induces strains in the structure of the printing form which creates a defect, called waves, in the resulting relief element. It is also desirable to keep the printing form firmly against the drum surface, particularly if the drum surface is cooled, to prevent or minimize heating of a back side of the printing form and to keep a support on the backside below its glass transition temperature. If the support of the printing form is heated to or above its glass transition temperature during thermal processing, the support can distort and result in deformations in the final processed relief printing form. Further, the printing form can be securely mounted in contact with the drum surface by having the printing form held tautly or in tension to the drum. But the printing form should be properly tensioned during thermal processing since the printing form can change in one or more dimensions when heated. In some instances, the printing form can expand in length when it is heated and adjustments are needed to keep the printing form taut to the drum surface. As such, the system should be capable of adjusting so as to maintain the desired tension on the printing form throughout thermal processing and properly remove of the unpolymerized portions from the relief surface as it is generated. If the printing form is not properly tensioned around the drum, and there is too little tension, the form may pull away from the drum and the unpolymerized portions may not be thoroughly removed. If the tension on the printing form is too high, the printing form can permanently distort or end portions of the printing form that attach to the drum can be damaged. Additionally, if the printing form is not appreciably held in contact with the drum, the printing form can rub against other parts of the processor and damage the relatively soft printing surface.
Non-uniform strains imparted in the printing form by not sufficiently securing the printing form to the drum surface during thermal processing, result in deformations that remain after the printing form has cooled or returned to room temperature. The deformations are waves of localized distortions resulting in a non-planar topography of the printing form. Waves of distortions can form in different locations in each printing form processed. Relief printing forms having waves result in poor print performance. In multicolor printing, when one or more of the relief printing forms have waves the printed image has poor registration. Even in single color printing, waves in the relief printing form may print an image that is not an accurate reproduction of its original, so called image infidelity, by printing straight lines as curves for example. Further, the relief printing form having waves may incompletely print the image due to intermittent contact of the inked surface of the printing form to the printed substrate.
Although there have been attempts to design systems for securing printing forms around the circumference of rotatable drums and to maintain tension on the printing forms, such systems have proven to be unsatisfactory. The inventions disclosed herein related to new and improved systems for securing a printing form about the circumference of a rotatable drum and maintaining tension on the printing form while it is secured about the circumference.
One example of a printing form is a relief printing plate, and in particular a flexographic printing plate. The invention disclosed herein can be used with flexographic plates, as well as other printing forms. Flexographic printing plates are well known for use in printing on soft and relatively hard materials, such as packaging materials, e.g. cardboard, plastic films, aluminum foils, etc. Flexographic printing plates can be prepared from photosensitive printing forms containing photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. DuPont has sold flexographic plates under the tradename CYREL® and has sold equipment for preparing the flexographic plates under the tradename CYREL® FAST. Equipment for preparing the flexographic plates may have a rotatable drum about which the flexographic plate is disposed, and the inventions disclosed herein can be used to secure the flexographic plate about the circumference of such drums and to maintain tension on the flexographic plates.
For example, processors for thermal development of flexographic printing forms may have rotatable drums about which flexographic plates must be secured. One such processor is described in U.S. Pat. No. 5,279,697. As disclosed in U.S. Pat. No. 5,279,697, an automated process and apparatus can be used for handling an irradiated printing form and heating and pressing the print form to remove the unirradiated composition from the printing form. As disclosed in U.S. Pat. No. 5,279,697, the printing form is secured to a preheating drum with a clamp flush mounted transversely on an outer surface of the drum.
Another method and apparatus for thermal processing a printing form is disclosed in U.S. Pat. No. 6,797,454 B1. The apparatus disclosed in the publication includes a drum having a metallic surface that is coated with a thin tackification coating that temporarily adheres the photosensitive printing form to the drum. Although the drum includes a tacky surface to hold the plate to the drum, the degree of tack at times can be insufficient to prevent the printing form from slipping or moving from the drum surface.
Similarly, in a commercial thermal processor sold under the tradename CYREL® FAST 1000TD, an exterior surface of the drum includes a silicone rubber layer having a Shore A hardness of 50, and a tackification layer on the rubber layer. The tackification layer is DOW 236, a silicone dispersion in solvent, available from Dow Corning, and it adheres the printing form to the drum.
This invention relates to new and improved systems and methods for securing printing forms on a base member for supporting the printing form, and in particular around the circumference of a rotatable drum. The inventions disclosed herein can generally be used in the processors described above as well as other processors and printing machines that process other printing forms. The inventions disclosed herein also include methods and systems for maintaining proper tension on the printing form after it has been secured around the circumference of a rotatable drum.